The present invention relates to plasma spray coating of parts, and more particularly concerns continuous process spray coating in a controlled environment.
Plasma spray coating of parts is often performed by an electric torch that generates a plasma stream of very high velocity and temperature. Materials, in the form of fine powder, that are to be spray coated upon a part are injected into a high velocity plasma stream and caused to impinge upon the part to provide the desired coating. An example of an electric arc plasma spray gun for use in such a process is shown in U.S. Pat. No. 3,914,573. It is often desired to carry out such plasma spray coating in a reduced oxygen or other controlled environment, so as to minimize oxidation or other chemical changes of the powder and part in the spraying process, or to confine toxic vapors. U.S. Pat. No. 3,839,618 illustrates such a reduced pressure or controlled environment plasma spraying operation.
Vacuum chamber plasma spray coating, for production of large quantities of sprayed parts, is presently performed by batch-type processing. For example, in spray coating turbine blades, a number (such as one hundred) of blades are mounted on a carrier which is placed in the chamber. The chamber is then sealed and evacuated, and the parts are preheated and sprayed, generally one at a time. In the process, the parts are heated to several thousand degrees Fahrenheit, but after a part is coated, it must remain on the carrier within the chamber until all other blades in the batch have been coated. The heated and coated parts, and their supports, act as massive heat sinks within the chamber, retaining a substantial amount of the heat of the plasma spray, and tending to cause an undesired buildup of heat within the chamber. Complex and expensive cooling systems are required to remove such heat from the chamber.
Not only does the batch spray coating process present serious heat control problems in a low pressure chamber, but the batch processing itself is inherently slow. Time is required in such batch processing to load and unload the parts and to repeatedly evacuate the relatively large volume of the processing chamber. During such loading, unloading, and chamber evacuation, no spraying or preheating can occur. According to presently known batch processing systems for electric arc plasma spray coating, a total of ten hours may be required to spray coat one hundred turbine blades, providing the slow production rate of approximately ten blades per hour.
Accordingly, it is an object of the present invention to provide for plasma spray coating of parts with a process and apparatus that substantially minimizes above-mentioned problems.